1. Field of the Invention
The present invention relates to a control device for a dc motor supplied with power by a voltage generator through a line having a given resistance, in which is inserted at least one relay controlling the motor in one of its directions of operation.
More precisely, this device has for purpose to control one of the mechanical magnitudes delivered by the motor, namely the torque or the speed, or the angular position of the output shaft, or a magnitude derived from these magnitudes. In some cases, the magnitudes delivered by the motor are of intermediate magnitudes, the effective purpose being to control a mechanism driven by the motor.
2. Description of the Related Art
Such control devices are known for certain equipments of vehicles, such as the sliding elements (window raiser or sliding roof) in respect of which it has already been proposed to improve their safety by control means whereby it is possible to interrupt the closing movement when the opening element encounters a foreign body, such as a finger, member or neck of an occupant (individual or animal). A known means consists in an electronic supervision of the magnitude of the current of the motor: when the current magnitude reaches a threshold exceeding the normal operating value, the motor is stopped and reversed so as to release the foreign body.
It has been proposed to improve this method by supervising not only the magnitude of the current, but also its variation with respect to time so as to anticipate the interruption of the operation before reaching a critical threshold.
It has been found that these devices do not correctly guarantee that a foreign body such as a finger, member or neck, is not injured. Indeed, if on the other hand it is desired to avoid accidental interruptions, the current threshold or current variation thresholds with respect to time must be placed rather high so as to avoid intefering with the normal variations due to current anomalies met with in sliding opening elements, such as: stiff points in the joints, anomalies due to frost, etc.
Furthermore, the electric conditions vary considerably with the voltage of the generator and the temperature (which affects the resistance of the circuit), and the thresholds to be determined become incompatible with an operation with no accidental interruption.
In view of these difficulties, it has proposed to add to the motor a sensor which enables the electronic supervising device to measure at any moment the speed of the motor and thereby avoid the problems due to the variations in the electrical conditions. In a known structure comprising such a sensor, the spectrum of the speed in normal operation of the opening element, is recorded in such manner as to constitute a reference with which the real operation is then compared. If the performance in the real operation excessively deviates from this reference, this signifies that a foreign body is in the path of the closing element so that the operation is interrupted and the motor reversed. Such a device has the drawback of being more complex owing to the fact that the sensor must be associated with the motor, and above all of failing to take into account normal variations which may be met with, above all after long periods of inactivity.
An object of the invention is therefore to solve the problem by taking into account variations in the electrical conditions and in particular those of the resistance of the circuit.